Chemical brightening of aluminum



Patented May 18, 1954 CHEMICAL BRIGHTENING OF ALUMINUM Roy C. Spooner, Kingston,

signor to Aluminium Montreal, Quebec, Can

Canada Ontario, Canada, as-

Laboratories Limited, ada, a corporation of No Drawing. Application June 29, 1950, Serial No. 171,235

8 Claims.

This invention relates to the chemical brigh ening of aluminum, also called chemical polishing. A particularly effective brightening procedure includes subjecting the aluminum sheet or other article to the action of an aqueous phosphoric-nitric acid solution, e. g. a bath comprising water, phosphoric acid and nitric acid, and sometimes supplemental ingredients such as acetic acid.

It has been found, however, that the brightening treatment eiiected in a bath of the abovementloned character is accompanied by vigorous dissolution of the metal surface. Thus there is a rapid buildup of aluminum salts in the solution. As a result, the bath must soon be discarded in favor of a fresh solution; for example, objectionable decrease of brightening effect is usually found to occur when the concentration of aluminum salts reaches 20 to 25 g. p. 1. (grams per liter). As the brightening operations proceed, some decomposition of the nitric acid also occurs, with disadvantage in that the desired results often depend on avoiding any great departure of the nitric acid concentration from a selected value.

It is therefore an important object of the invention to increase the life of a given bath by decreasing the rate of attack on the aluminum surface during the brightening process. Other objects are to provide an improved brightening method for aluminum, wherein metal loss may be decreased without serious effect on the brightenin operation, and more generally, to provide new and efficient procedure wherein one or more of the disadvantages mentioned above may be lessened or delayed.

Further objects and advantages will appear in the course of the following description.

The invention may be briefly stated as embracing the discovery that silicic acid, when added to a bath of the stated character, constitutes an eifective and nondetrimental inhibitor. Specifically, it has been found that such addition substantially decreases the loss of metal from the surface of the aluminum during the brightening process without a corresponding decrease in brightening action and Without any other eiiects that might be objectionable. Indeed there is indication that the silicic acid also acts to retard the decomposition of the nitric acid, and thus to prevent a rapid change in the concentration of the latter as the bath is used.

In practice of the invention, silicic acid is incorporated in the mixture of concentrated reagent acids, wherein it is found to be fully sol- 2 g uble, in proportions of the order indicated below and at the elevated temperatures preferred for operation. It will be understood that these concentrated acid mixtures are highly viscous, particularly at low temperatures. For the brightening process, the bath is brought to and maintained at a high temperature, usually about to C. but preferably no higher than necessary for successful results. The aluminum articles are then treated by inserting them in or otherwise exposing them to the complete liquid composition (which is fluid, though viscous, in its heated state) for a suitable length of time, usually about two to five minutes, the workpiece or the solution or both being suitably agitated during this period; during this interval the brightening or polishing action takes place at the surface of the article, producing not only a cleaning effect but a marked improvement in the brightness of the surface, measurable as an increase in the mirror-like or specular properties of reflection. The time required for the treatment may vary with circumstances and results desired. In many cases two minutes is sufiicient, Or occasionally as little as 30 seconds, While it is at present believed that advantage is seldom had in carrying the operation beyond five minutes or so, except perhaps to compensate for preliminary or other reduction of the bath temperature when large objects or pieces are being treated. In general the time should be no longer than necessary to achieve a useful brightening effect, since even with the inhibiting action in the present process, dissolution of metal and modification of the bath composition are found to continue progressively as long as the metal is exposed in the bath.

The amount or silicic acid may vary considerably, in that good results have been obtained over a range of about 5 grams to 50 grams or $0, per liter of the reagent acid solution, indication being that useful reduction in metal (i. e. aluminum) loss and in consequent contamination of the bath can sometimes be had with the silicic acid in an amount aslittle as one gram per liter. In some instances, the metal loss has been lower at the higher concentrations of the inhibitor, yet the effect of the inhibitor on the actual brightening action seems to b advantageously slight over a Wide range of concentrations. Indeed it appears that relatively very large amounts, say up to grams/liter, can be used in some cases, but in general, for reasons of economy, the silicic acid content should be as low as will permit attainment of the desired results. Although the action of the silicic acid (in impeding dissolution panels of aluminum of metal and otherwise stabilizing the bath) seems quite evidently to be of a chemical sort rather than as a merely physical diluent, the precise nature of the action has not been ascertained. Nevertheless, the effectiveness of silicic acid in achieving the stated results has been abundantly demonstrated by test, in contrast to a number of other substances which at most merely delayed or impeded the activity of the bath in its brightening as well-as other reactions.

In one series of operations, which may serve as examples to illustrate the invention, the basic brightening solution had the following composition, by volume:

Parts Orthophosphoric acid (85%) 80 Concentrated nitric acid 5 Glacial acetic acid 15 To portions of this solution various quantities of silicic acid were added, and pieces of aluminum were then treated with such portions, by immersion for various periods of time, at temperatures chiefly in the range of 90 to 95 C. For comparison, treatments of like pieces of aluminum were also performed in the basic solution without the inhibitor. The metal treated was a standard aluminum alloy sheet having a customary commercial finish, i. e. as usually characterizes the mill product. Specifically, the brightening test operations were performed on alloy 38 mill finish sheet, eachpanel having a total surface (counting both sides) of about 64.5 square centimeters. The brightening effect was specular reflectance, which for the original, 1. e. mill finish sheet, before these treatments, had an average value of 35 reflectance units. The following Table I shows the results of test operations under various conditions, including also tests without silicic acid, and showing in all cases the relative specular reflectance of the treated panel and the weight loss exhibited by the treated panel, such less being taken as a measure of the amount of metal dissolved in the bath and there remaining as contamination, i. e. in the form of aluminum salts.

Table I (1 tion irne o ure ure a ve Silicic Acid, Treatment ing Treat- Specular g grams/liter (min) li lelt, Reflectance dicating a reduction in the high rate of decomposition of nitric acid which .is usually experienced with these baths.

Another set of operations, by way of further example of the invention, was performed with a bath having the following composition: HaPOr 94%, HNOa 6% by volume, the acids being of the measured by the relative same concentrated character as mentioned above. Th aluminum articles treated were sheet panels of similar nature, each having a total surface area of about 64.5 square centimeters. Each operation involved the immersion of a number of such panels in the bath for 2 minutes, at a bath temperature of 99 C. The results, with baths containing no inhibitor and containing various amounts of silicic acid measured as grams per As in Table I, the operations with the inhibitor provided a marked decrease in loss of metal to the bath, yet only small decrease in the brightness of the treated surfaces. Indeed the results of all tests may be generally characterized as yielding a percent of decrease in weight loss of the aluminum that is notably greater than the percent of decrease in brightening efiect. Since even as diminished, the brightness or state of polish of the treated aluminum is ample for commercial requirements, the present process thus represents a very considerable practical advantage, in providing a markedly lower loss of metal and in permitting longer use of a given bath of the character described.

While from the above examples optimum conditions of operation may be readily selected from the specific typesof bath and character of work then tested, it will be appreciated that the times ,of treatment, proportions of inhibitor and other operating values to be'chosen in other cases may be very easily and rapidly determined, if necessary by simple test of the sort indicated above, so as to yield best results in any given set of circumstances. As implied in the foregoing description the term aluminum is used herein to mean both the pure metaland aluminum base alloys, such as those of customary commercial character, since the invention appears equally applicable to various sheets, shapes or other articles which are essentially or in major part composed of aluminum.

It is to be understood that the invention is not limited to the specific embodiments hereinabove described but may be carried out in other ways without departure from its spirit.

I claim:

1. The method of brightening aluminum which comprises subjecting the same to a hot, concentrated phosphoric and nitric acid brightening bath which contains a major proportion of phosphoric acid and a minor proportion of nitric acid, and inhibiting the metal-dissolving chemical action of the bath on the aluminum relative to the attainment of a desired brightness of the aluminum under said chemical action, by incorporating silicic acid in said bath.

2. The method of claim 1, wherein the bath also includes a minor proportion of acetic acid.

3. The method of decreasing loss of metal from the surface of aluminum during brightening treatment with a hot concentrated phosphoric and nitric acid bath, which consists in incorporating silicic acid in amount of 1 to grams per liter in the aforesaid brightening bath that contains a major amount of phosphoric acid and a minor amount of nitric acid, and exposing the aluminum to the resulting hot composition for brightening action thereof.

4. The method of brightening aluminum which comprises subjecting the aluminum to brighten ing action of a hot, concentrated phosphoric and. nitric acid brightening bath in which phosphoric acid is the major constituent and which contains a small amount of silicic acid for inhibiting the metal-dissolving chemical action of the bath.

5. The method of brightening aluminum which comprises subjecting the same to a hot, concentrated phosphoric and nitric acid brightening bath that contains a major amount of phosphoric acid and a minor amount of nitric acid, and inhibiting the metal-dissolving chemical action of the bath on the aluminum relative to the attainment of a desired brightness of the aluminum under said chemical action by incorporating in said bath a minor amount of silicic acid.

6. The method of claim 5, wherein the bath also contains a minor amount of acetic acid.

7. The method of claim 3, wherein the bath also contains a minor amount of acetic acid.

8. The method of claim 4, wherein the silicic acid is present in amount of 5 to 50 grams per liter of the bath.

References Cited in the file Of this patent UNITED STATES PATENTS Number Name Date 2,146,83-8 Newsome Feb. 14, 1939 2,446,060 Pray et a1 July 27, 1948 2,465,750 Reid Mar. 29, 1949 2,572,497 Law Oct. 23, 1951 OTHER REFERENCES Mears et a1., Use of Inhibitors for Aluminum, Reprint 8316, published by Electrochemical Society-paper presented April 8, 1943 (pages 203 and 204). 

1. THE METHOD OF BRIGHTENING ALUMINUM WHICH COMPRISES SUBJECTING THE SAME TO A HOT, CONCENTRATED PHOSPHORIC AND NITRIC ACID BRIGHTENING BATH WHICH CONTAINS A MAJOR PROPORTION OF PHOSPHORIC ACID AND A MINOR PROPORTION OF NITRIC ACID, AND INHIBITING THE METAL-DISSOLVING CHEMICAL ACTION OF THE BATH ON THE ALUMINUM RELATIVE TO THE ATTAINMENT OF A DESIRED BRIGHTNESS OF THE ALUMINUM UNDER SAID CHEMICAL ACTION, BY INCORPORATING SILICIC ACID IN SAID BATH. 